Naturally, asymmetries can be found in humans as well as horses. When it comes to horses, the typical forms of structural asymmetry are in bone size, muscular development and hoof dimensions. It has been noted by Sarah Jane Hobbs, et al, in their recently-published study that about 5% of Dutch Warmbloods have uneven dorsal hoof wall angles in the front limbs. The development of this unevenness can often be attributed to horses favoring one side or the other, often termed sidedness.
Hoof unevenness has also been associated with certain grazing patterns, which are another manifestation of sidedness. Warmblooded foals that have long legs and short heads have difficulty reaching the ground to graze so they habitually graze with one forelimb stretched forward and the other one stretched backward. This posture eventually results in a lower angle in the protracted hoof and a higher angle in the retracted hoof. Training may also affect sidedness. The fact that they are habitually led on the left side might indicate why 90% of Thoroughbreds, Arabians and American Quarter Horses chose right handedness patterns in the largest equine sidedness study, according to Hobbs. This may lead to preferential loading of the right limb to avoid stepping on the trainer’s feet. Another mechanism for development of uneven hoof angles occurs in lame horses. In order to off-load the painful limb, the horse must over-loads the other limbs. Eventually, unevenness develops with the painful limb having a narrower, more upright hoof and the compensating limb having a wider, flatter hoof.
However, according to Hobbs, “compensatory mechanisms and out of plane ground reaction forces (GRFs) due to having uneven hooves had not previously been documented, so the aim of this study was to investigate the effects of limb specific (left vs. right) and directionally-based (high vs. low) fore hoof unevenness on contralateral fore and hind limb force vectors patterns, in both sagittal and frontal planes.” The goal is to seek a link between uneven dorsal hoof wall angles in the forelimbs and asymmetrical load-bearing issues in the hind limbs.
This study consisted of 24 riding horses divided into groups. Horses with fore hoof angles that differed by less than 1.5 degrees were considered EVEN, of which there were 7. Those in the UNEVEN group had a difference between left and right fore hoof angle of 1.5 to 12.5 degrees. These horses were then divided into subgroups based on whether the hoof angle was higher on the left or right side, the HIGH-LF (left fore) group had 12 horses and the HIGH-RF (right fore) group had 15 horses. At the start of the study, dorsal hoof wall angles were measured. The horses were evaluated for lameness in hand at walk and trot. None of the horses showed signs of lameness at trot but 9 horses (5 HIGH-LF, 4 HIGH-RF) were described as showing a Grade 1 lameness at walk that was described as being mechanical rather than painful in origin.
Researchers applied 6 retroreflective markers per hoof and used high-speed cameras to track the motion of the horse as it trotted in hand along a rubberized runway with an embedded pressure mat and force plate. Trials were repeated until at least three clean force plate hits had been recorded for each of the 4 limbs. The data for the HIGH-LF and HIGH-RF groups were compared using a novel method of force vector analysis.
The team proposed that the left/right forces in the front limbs would be influenced by the degree of asymmetry between the dorsal hoof walls. The data supported this hypothesis with the higher hoof providing more forward propulsion than the lower hoof. The discrepancy in propulsive forces increased with the degree of asymmetry between hooves called Pedotti diagrams. An important finding was that asymmetries associated with having a high-low fore hoof conformation were not mirror images for HIGH-LF vs HIGH-RF.
In the HIGH-LF group, a greater discrepancy between hoof angles was associated with an increase in propulsive force in the right hind limb. As a result, the LF-RH diagonal pair provided more forward acceleration than the RF-LH diagonal pair.
In the HIGH-RF group, the force vector was directed toward the left in all limbs. This would facilitate turning left but would be expected to make it more difficult for the horse to turn right, so the horse would be more likely to slide to the outside on a turn or circle to the right.
This study builds upon knowledge of the biomechanics of uneven-footed horses by showing that changes associated with having a higher LF or higher RF are not mirror images of each other. Horses that are HIGHER-RF have differences in mediolateral forces that may have implications for the orthopedic health of the horse.